Chlorinated solvent movement in River Terrace Gravels Debbie - PowerPoint PPT Presentation

Chlorinated solvent movement in River Terrace Gravels Debbie Wilkinson National Environment Programme Restoring Sustainable Abstraction Investigation & Groundwater Catchment Management Lead May 2019

Agenda • Background to Tonbridge Ø Location Ø Geology Ø Chlorinated Solvents • Monitoring Network • Conceptualisation (simple) • Chlorinated solvent movement Ø Seasonal Variations / Water Levels Ø Abstraction Ø Degradation (natural attenuation) Above: Tonbridge Water Treatment Works • Options Appraisal (former Amec Foster Wheeler) • Conclusions 2

Background to Tonbridge - Location Site details: • Tonbridge WTW is located in Tonbridge, Kent. • Adjacent to the River Medway. • The catchment is semi-urban with the town of Tonbridge to the east and rural land to the west. • South East Water is licenced to abstract up to 4Ml/d from the Gravel Aquifer. Above: Location and Source Protection Zone (SPZ) Map for Tonbridge Water Treatment Works. Red = SPZ 1; Green = SPZ 2; Blue = SPZ 3 3

Background to Tonbridge - Geology Geology Description Aquifer Drift - Alluvium Low permeability silty Non-aquifer clay (up to 6m thick) Drift - River Brick earth gravels (up Secondary A Terrace Gravels to 4m thick) aquifer Solid - Tunbridge Fine sands, sandstone Secondary A Wells Sands and clay (up to 12m aquifer thick) Solid - Wadhurst Clay and shale with Non-aquifer Clay subordinate sandstone, (aquitard) ironstone and limestone (up to 60m thick) Solid - Ashdown Interbedded sandstone Secondary A Beds and limestone (70m aquifer plus thick) Above: Cross-section from Halcrow (2006). 4

Background to Tonbridge - Geology Comprises of five shallow radial wells that abstract water from the River Terrace Gravel aquifer. Above and Left: Drilling of new gravel Ranney (radial) wells at Tonbridge in 2015. 5

Background to Tonbridge - Chlorinated Solvents • Since mid 2000’s, chlorinated solvents have been detected in groundwater including Tetrachloroethene (PCE), Trichloroethene (TCE), and lower concentrations of 1,2-Dichloroethane (1,2 DCA). Ø Uses of chlorinated solvents include: dry cleaning, degreaser for metals, paint striping, printer inks etc… • Highest concentrations of chlorinated solvents are found in Gravel Well 1, which is located closest to the town of Tonbridge. • Water abstracted from Gravel Well 1 is required to be blended with other sources to meet the Drinking Water Standard (10 µg/l). 6

Chlorinated Solvents – Monitoring Network • Environment Agency drilled several observation boreholes in 2011-12 in the urban area of Tonbridge (shown in red) to investigate the source of chlorinated solvents. • GS1 to GS5 are South East Water’s production Gravel Wells. • Blue hatched area is the Groundwater flow capture zone for Gravel direction under non- Well 1 (GS1) (under pumped conditions pumped conditions). 7

Conceptualisation 8

1. Chlorinated Solvents – Seasonal Variations / Water Levels Chlorinated solvent concentrations increase each year in the autumn months and decrease following winter recharge to the River Terrace Gravel aquifer, however, there is a notable lag effect. 9

2. Chlorinated Solvents – Abstraction Gravel Well 1 (GS1) The abstraction regime affects chlorinated solvent concentrations at Gravel Well 1 • as abstraction rate increases, after a lag period, chlorinated solvent concentrations also increase • as abstraction rate ceases, after a lag period, chlorinated solvent concentrations decrease • when abstraction rates are stable chlorinated solvent concentrations are stable Observation Boreholes An increase in chlorinated solvent concentration at observation boreholes under pumping and/or non-pumping conditions may help indicate the likely source of the chlorinated solvents; • if under pumping conditions chlorinated solvents increase, then the likely source is up-gradient • if under pumping conditions chlorinated solvents decrease, then the likely source is down-gradient • if concentrations remain stable under all conditions it is likely that they do not lie on a flow path from the source Uncertainties include: • a time lag • the natural flow direction under non-pumped conditions might not be 180 degrees (west to east) 10

Chlorinated Solvents – Abstraction ( Under Pumped Scenario ) 11

Chlorinated Solvents – Abstraction ( Under Non-Pumped Scenario ) 12

3. Chlorinated Solvents - Degradation Under favourable conditions chlorinated solvents will undergo degradation (natural attenuation). Favourable conditions are: • reducing conditions • where there is organic matter, or; • other electron donors Evidence that degradation is occurring is through the presence of daughter products. In the case of PCE daughter products include: • TCE • 1,2 DCE • Cis 1,2 DCE • Vinyl Chloride Monitoring has shown some increase in daughter products (e.g. cis 1,2 DCE) in the more contaminated observation boreholes (e.g. A1), but not at significant ratios. The persistence of chlorinated solvents in the River Terrace Gravel aquifer suggests unfavourable conditions for wider degradation to occur (possibly due to the lack of electron donors). 13

Chlorinated Solvents – Options Appraisal A number of remediation techniques were appraised over the suitability to reduce chlorinated solvent concentrations in the River Terrace Gravels and at the Tonbridge Gravel Well abstractions. Remediation techniques appraised included : Technique Description Containment hydraulic barriers: Abstraction of groundwater is used to prevent the movement of contaminated groundwater. Containment in-ground barriers: A low permeability barrier is installed in a trench to prevent the moment of contaminated groundwater. Monitored Natural Attenuation: The natural degradation process of chlorinated solvents without intervention. Enhanced anaerobic An organic medium is added to the aquifer to produce reducing conditions for chlorinated solvents to degrade. De- halogenating bacteria may also be required for the process to work effectively. biodegradation: Phytoremediation: Involves the uptake of chlorinated solvents via plant roots. Chemical oxidation: Oxidants are added to the aquifer / groundwater to oxidise the contaminants. Oxidants could include ozone, hydrogen peroxide, calcium peroxide, sodium persulfate, and sodium / potassium permanganate. A reductant, e.g. Zero Valent Iron is added to the aquifer. This reacts with the chlorinated solvents to break them down to less Chemical reduction: toxic daughter products. Dual phase soil vapour extraction: Uses a vacuum system to remove contaminated groundwater from the subsurface. Air sparging / bio sparging: Involves injecting air into the contaminated aquifer. Permeable reactive barrier: In-ground barrier that contains reactive material through which groundwater can flow through. The contaminants are broken down via the reactive material. Pump and treat: The contaminated plume is captured by the abstraction wells, pumped to the surface and treated. Thermal methods: Involves increasing the groundwater temperature to aid and accelerate chemical reactions. In situ flushing: Involves flooding a zone of contaminated land to remove contaminant from the soil. Additional treatment: Additional treatment at Tonbridge Water Treatment Works. Do nothing. Do nothing: 14

Chlorinated Solvents – Preferred Option The preferred option was In Situ Chemical Reduction (ISCR) using Zero Valent Iron (ZVI) in a permeable reactive barrier due to: • groundwater conditions in the River Terrace Gravels already appear to be mildly reducing with some naturally occurring background iron and manganese concentrations • technical effectiveness and durability, compared with the second preferred option of Enhanced Anaerobic Biodegradation. Above : Example of In Situ Chemical Reduction (ISCR) 15

Conclusions Chlorinated solvent fluctuations and movement in the aquifer is complex, due to: Ø seasonal changes and groundwater recharge, and; Ø the effects of an intermittent abstraction regime on groundwater flow causing dispersion and dilution of the chlorinated solvents. Likely source of PCE is east of observation borehole A1 and west of observation boreholes C1 and EN04. No free phase of chlorinated solvents have been found. Degradation (natural attenuation) has been shown to be ineffective, especially over reasonable timescales. The options appraisal identified that In Situ Chemical Reduction (ISCR), using Zero Valent Iron (ZVI) was the preferred option, particularly due to technical effectiveness and durability, but also that the aquifer is naturally mildly reducing. Next Steps • Continue monitoring and refine the hydrogeological conceptualisation. • Undertake stakeholder engagement to promote best practice of the storage and handling of chlorinated solvents. • Feasibility study for ISCR using ZVI, leading to pilot study, if successful… 16

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